1. Field of the Invention
The invention relates to a cathode ray tube comprising
an electron source having a cathode for emission electrons,
an electron beam guidance cavity having an input aperture and an output aperture, said cavity having walls, at least a part of the wall of the electron beam guidance cavity near the output aperture comprising an insulating isolating material having a secondary emission coefficient xcex41 for cooperation with the cathode, and
a first electrode connectable to a first voltage source for applying, in operation, an electric field with a first field strength E1 between the cathode and the output aperture, xcex41 and E1 having values which enable electron transport through the electron beam guidance cavity.
2. Description of the Related Art
Such a cathode ray tube is known from U.S. Pat. No. 5,270,611 which describes a cathode ray tube is described which is provided with the cathode, the electron beam guidance cavity and the first electrode connectable to a first voltage source for applying the electric field with a first field strength E1 between the cathode and the output aperture. Furthermore, the secondary emission coefficient xcex41 and E1 have values which enable electron transport through the electron beam guidance cavity. The electron transport within the cavity is possible when a sufficiently strong electric field is applied in a longitudinal direction of the electron beam guidance cavity. The value of this field depends on the type of material and on the geometry and sizes of the walls of the cavity. The electron transport then takes place via a secondary emission process so that, for each electron impinging on a cavity wall, one electron is emitted on average. The circumstances can be chosen to be such that as many electrons enter the input aperture of the electron beam guidance cavity as will leave the output aperture. When the output aperture is much smaller than the input aperture, an electron compressor is formed which concentrates the luminosity of the electron source by a factor of, for example, 100 to 1000. Such a cathode ray tube may be used in television display devices, computer monitors and projection TVs.
The electron beam current of the known device can be modulated by a variation of the voltage supplied to the first electrode.
A drawback of the known device is that the modulation voltage on the first electrode must be relatively high. For example, a modulation voltage of 200 volts is necessary for modulating of a current between 0.1 and 2 mA. Therefore, relatively expensive high-voltage electronics is required for the driving circuits of the cathode ray tube.
It is, inter alia, an object of the invention to provide a cathode ray tube in which the electron beam current is modulated with a relatively low voltage. To this end, the cathode ray tube according to the invention is characterized in that the cathode ray tube comprises a second electrode placed between the cathode and the cavity, the second electrode being connectable to a second voltage source for applying, in operation, an electric field with a second field strength E2 between the cathode and the second electrode for controlling the emission of electrons. The invention is based on the recognition that, by placing the second electrode between the cathode and the input aperture of the electron beam guidance cavity, the pulling field near the cathode is determined by the applied voltage on the second electrode, and hence the electron beam current can be modulated. In this way, the second electrode enables modulation of the current leaving the electron beam guidance cavity with a relatively low positive voltage difference, for example, in a range from 1 to 10 volts, with respect to the cathode, when the distance between the second electrode and the cathode is small enough. Low-cost, low-voltage electronics can thus be applied in the driving circuits of the cathode ray tube. A further advantage is that the influence of modulation on the characteristics of the electron beam leaving the electron guidance cavity is reduced by applying the modulation voltage on the second electrode. The characteristics of the electron beam are, for example, spot size and velocity distribution of the electrons.
A particular version of the cathode ray tube according to the invention is characterized in that the second electrode comprises a gauze. An effective pulling field can thus be established, which directs the electrons to the input aperture of the electron beam guidance cavity.
A further embodiment of a cathode ray tube according to the invention is characterized in that the second electrode comprises an electrically conductive cavity having an inlet and an outlet, the inlet facing the cathode and the outlet facing the input aperture of the electron beam guidance cavity, the inlet being covered with the gauze for creating, in operation, an electric field-free space in the conductive cavity.
A further embodiment of a cathode ray tube according to the invention is characterized in that the electrically conductive cavity comprises a hollow, conductive cylinder. In this way, the field-free space is extended within the cylinder, and the influence of the transport electric field in the electron beam guidance cavity on the emission of electrons from the cathode is further reduced.
A further embodiment of a cathode ray tube according to the invention is characterized in that a distance between the cathode and the second electrode is in a range between 20-400 micrometer. For example, when the distance between the cathode and the second electrode is 100 micrometer, an amplitude modulation of 5 Volts is sufficient for modulating a current between 0 and 3 mA when conventional oxide cathodes are used.
A further embodiment of a cathode ray tube according to the invention is characterized in that the cathode is positioned eccentrically with respect to the output aperture of the electron beam guidance cavity. This position of the cathode prevents electrons coming from the cathode from travelling to the output aperture of the electron beam guidance cavity along a direct path, thus without interaction of the walls of the electron beam guidance cavity. The electrons that pass through the output aperture of the electron beam guidance cavity, without interaction with the walls thereof, may be disadvantageous to the electron beam characteristics of the electrons emitted from the electron beam guidance cavity.
A further embodiment of a cathode ray tube according to the invention is characterized in that the cathode ray tube comprises shielding means placed between the cathode and the output aperture to prevent electrons from travelling along a direct path from the cathode to the output aperture. This shielding means also prevents electrons coming from the cathode from travelling to the output aperture of the electron beam guidance cavity along the direct path between the cathode and the output aperture, without interaction of the walls of the electron beam guidance cavity.
A further embodiment of a cathode ray tube according to the invention is characterized in that the gauze comprises a shield plate having a diameter which is at least equal to that of the output aperture of the electron beam guidance cavity, a center of the shield plate being placed axially with respect to a center of the output aperture to prevent electrons from travelling along a direct path from the cathode to the output aperture.
A further embodiment of a cathode ray tube according to the invention is characterized in that the electron beam guidance cavity comprises a body having dimensions which are at least equal to that of the output aperture of said cavity, the body comprising an insulating material having a secondary emission coefficient xcex42, xcex42, and E1 having values which enables electron transport along the body towards the output aperture, the body being placed axially with respect to the output aperture.
The secondary emission coefficient xcex42 of the insulating material used in the body may have the same value as the secondary emission coefficient xcex42 of the insulating material used in the electron beam guidance cavity. In this way, the possibility that electrons will directly travel from the cathode to the output aperture without interactions is reduced, and the efficiency of the cathode structure is increased as compared with a cathode structure that uses a shield plate.
A further embodiment of a cathode ray tube according to the invention is characterized in that the cathode ray tube further comprises a filament for heating the cathode, the filament having first and second terminals, the first terminal being connectable to a positive terminal of a power supply means and the second terminal being connectable to a negative terminal of the power supply means, the second electrode being coupled to the first terminal and the cathode being coupled to the second terminal, a distance between the cathode and the second electrode and the applied voltage between the first and second terminal determining, in operation, the emission of electrons. The numbers of terminals of the cathode ray tube may thus be reduced, and only two terminals of the cathode ray tube are necessary to control the cathode, the second electrode and the filament. The voltage difference between the terminals of the filament determines the voltage difference between the cathode and the second electrode.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.